Image compositing devices generally mix a foreground subject against a blue or green screen or backing with a background image. The resulting video image is the foreground subject, with the blue or green screen replaced by the background image. Very high quality image compositing devices can reproduce shadows and transparency details such that the background image is linearly "turned on" by a control (key) signal at the exact level to produce a convincing composite. In this manner, shadows cast by the foreground subject on the blue or green backing appear, in the composite, to be cast on the background image. U.S. Pat. No. 4,100,569 discloses such an image compositing device.
However, high quality compositing devices which can reproduce shadows and delicate transparency details can also reproduce common lighting and lens problems. Backing screens are often lit from the top causing the top of the backing to be much brighter than the bottom. An automatic gain control circuit which evaluates the key signal generated from the screen "sees" the top of the screen as fully lit and the remainder of the screen as lower in illumination, in effect, causing the remainder of the screen to be defined as a shadow. The second problem is lens vignetting. Many times it is necessary to use lens polarizers in blue or green screen shooting to prevent glare. This produces a lower light level at the camera tubes and causes the camera operator to open the lens iris to compensate for the lower light level. When the lens is opened, however, the corners of the picture may become darkened which then appears in the composite as a shadow which really does not exist. It is possible to have both of these effects occur without being noticed until a composite is attempted. If the composite is done in post-production utilizing a previously produced foreground tape, there is no simple way to fix any such problems in the foreground tape. If the result must be used in a multiple layer application, each new layer multiplies the effect of the previous layer. If the original corners are only 2% darker than the rest of the screen, they will be 8% darker in the third layer pass. It is possible to increase the overall gain of the key signal to compensate, but the result produces a signal gain that is too high for the correctly lit portions of the screen. This excess gain produces a loss of detail and glowing edges around the subject of interest.